rjmccall added a comment. In D79279#2170187 <https://reviews.llvm.org/D79279#2170187>, @jfb wrote:
> In D79279#2170157 <https://reviews.llvm.org/D79279#2170157>, @rjmccall wrote: > > > I think the argument is treated as if it were 1 if not given. That's all > > that ordinary memcpy formally guarantees, which seems to work fine > > (semantically, if not performance-wise) for pretty much everything today. > > > I'm not sure that's true: consider a `memcpy` implementation which copies > some bytes twice (at different access size, there's an overlap because > somehow it's more efficient). That would probably violate the programmer's > expectations, and I don't think `volatile` nor atomic `memcpy` allow this > (but regular `memcpy` does). Yes, that's true, if you need an only-accessed-once guarantee, that's above and beyond just a minimum access size. I agree that `volatile` would need to make this guarantee. >> Do you think it'd be useful to have different guarantees for different >> operands? I guess it could come up, but it'd be a whole lot of extra >> complexity that I can't imagine we'd ever support. > > You mean, if `element_size` is passed then you get different guarantees? No, sorry, I mean different guarantees for the different pointer operands. In principle, we could allow you to say that the memcpy has to be done with 4-byte accesses from the source and 2-byte accesses to the destination. That's implementable but a lot of work. >> If one of the arguments is `volatile`, arguably the minimum access width (if >> given) needs to be exact. If we don't support that right now, it's okay to >> make it an error, which is basically you've already done with the `_Atomic >> volatile` diagnostic. > > Agreed. `volatile` with size makes a lot of sense, and the IR version of it, > once created, ought to not be able to widen accesses. `volatile` without a > size specified makes sense too, because you just want a single read and a > single write, don't care about tearing. Right. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D79279/new/ https://reviews.llvm.org/D79279 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
